Currently, SVQ1EncContext is defined in a header that is also
included by the arch-specific code that initializes the one
and only dsp function that this encoder uses directly.

But the arch-specific functions to set this dsp function
do not need anything from SVQ1EncContext. This commit therefore
adds a small SVQ1EncDSPContext whose only member is said
function pointer and renames svq1enc.h to svq1encdsp.h
to avoid exposing unnecessary internals to these init
functions (and the whole mpegvideo with it).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>